There are numerous cases where a thin-film transistor (TFT) having a top-gate structure is used as a switching element formed in each pixel formation portion of a liquid crystal display device. In such a TFT having a top-gate structure, a gate electrode and a gate oxide film are formed such that side surfaces are substantially vertical to a semiconductor layer. For example, Patent Document 1 discloses a TFT which is formed such that side surfaces of a gate electrode are substantially vertical to a semiconductor layer made of an oxide semiconductor. In this case, hydrogen contained in a silicon nitride film (SiNx) formed on the semiconductor layer spreads into the semiconductor layer, whereby low-resistance regions, which serve as source and drain regions of the TFT, are formed in the semiconductor layer. Moreover, Patent Document 2 describes a TFT having a top-gate structure with a gate electrode whose side surfaces are tapering.